1. Field of the Invention
This invention is directed to a process which utilizes fluosilicic acid, phosphate rock and sulfuric acid to produce wet process phosphoric acid and hydrogen fluoride and/or fluoride salts. In the process of the present invention, the fluosilicic acid is reacted with phosphate rock in order to obtain phosphoric acid and calcium fluoride which are thereafter reacted in a crystallizer with sulfuric acid to form a gypsum slurry which is filtered to obtain a weak phosphoric acid solution and hydrogen fluoride. The resultant solution is treated to remove the hydrogen fluoride after which the hydrogen fluoride is concentrated and/or converted to fluoride salts while the phosphoric acid may be removed or transferred to a conventional phosphoric acid plant reactor associated with a system for producing wet process phosphoric acid. The use of the transferred weak phosphoric acid to a conventional phosphoric acid plant reactor will increase the production of phosphoric acid in the commercial production of wet process phosphoric acid in the conventional process.
2. History of the Related Art
Wet process phosphoric acid is commercially produced by chemically attacking phosphate rock within a reactor with concentrated sulfuric acid in a medium of phosphoric acid and calcium sulfate (gypsum). The resulting phosphoric acid gypsum slurry is filtered under vacuum to separate the liquid phosphoric acid product from the solid gypsum waste. Two or more stage countercurrent washes on the gypsum filter are used to provide maximum recovery of water soluble P.sub.2 O.sub.5. The wash water and recovered acid are returned to the reactor to control acid concentration and percent solids. The reactor provides a vehicle for contact and reaction of the rock and sulfuric acid under the necessary conditions for the nucleation and growth of the gypsum crystals.
The above described reaction is carried out in one or more vessels each consisting of one or more agitated compartments. The process is based on the fundamentals of adding the phosphate rock and sulfuric acid to a large circulating mass of phosphoric acid and gypsum to provide uniform concentration throughout the reaction mass, constant reaction mass temperature, and proper crystal growth retention time in order to yield the highest attack and filtration efficiencies. The resulting filter or product acid containing dissolved impurities is further processed by evaporation to produce a more concentrated acid for sale or to produce other phosphate fertilizer products.
The reaction with phosphate rock, which is comprised primarily of tricalcium phosphate (Ca.sub.3 (PO.sub.4).sub.2), calcium carbonate (CaCO.sub.3) and calcium fluoride (CaF.sub.2), produces carbon dioxide (CO.sub.2) and hydrogen fluoride (HF) in addition to phosphoric acid and gypsum. The carbon dioxide evolves from the process while the hydrogen fluoride reacts with the silicon, or sand, left in the rock after beneficiation to produce silicon tetrafluoride (SiF.sub.4).
Fluorine evolves from the reaction step as silicon tetrafluoride and from the subsequent concentration step as silicon tetrafluoride and hydrogen fluoride. The distribution of fluorine from the manufacture of crude wet process phosphoric acid is as follows:
______________________________________ % of Total F ______________________________________ 1. The gypsum 10-20 2. Emissions from the reactor 10-25 3. Vapors produced during 40-60 concentration 4. The concentrated product acid 10-20 ______________________________________
The fluoride evolved during the reaction in conventional processes is typically absorbed into pond water in order to limit the quantity of fluorides emitted from the process so as to conform to existing environmental standards. The fluorine evolved during concentration steps is either recovered as fluosilicic acid (H.sub.2 SiF.sub.6) or is absorbed into the pond water used to condense the water vapor liberated during the evaporation process.
The number of phosphoric acid producers who recover fluorine as fluosilicic acid is limited. This is due to the relatively small demand of the acid for fluoridating drinking water with fluosilicic acid or its sodium salt, sodium silico fluoride (Na.sub.2 SiF.sub.6), and the manufacture of cryolite and aluminum fluoride.
Because of the small demand, the bulk of the fluorine evolved during the manufacture of wet process phosphoric acid is absorbed in the cooling pond. Fluorine is evolved from the pond water when it returns to the cooling pond resulting in a fluorine pollution problem. The fluorine level in cooling ponds builds up to about 4,000 ppm for producers who recover fluosilicic acid and to about 25,000 ppm for producers who do not. At these levels it is estimated that approximately two to twenty pounds of fluorine per day per acre of cooling pond surface is emitted. Normally the cooling ponds are 100 to 500 acres in size and the nonpoint source fluorine emission to the atmosphere is significant.
In order to overcome the problems of emissions of fluorine pollutants to the environment, the inventor of the present application designed and patented a closed loop system for the elimination of fluorine pollution from phosphoric acid plants as described in U.S. Pat. No. 3,811,246, the contents of which are incorporated herein by reference. Basically, the closed loop system for removing fluorine includes a process which involves a condensing of the vapors from phosphoric acid operations, especially from a phosphoric acid vacuum evaporator, by contacting the vapors in a scrubber with an aqueous liquid which absorbs fluorine vapors. During the process by-product fluosilicic acid is intermittently recovered while the remaining acid is recycled. As previously noted however, as the demand for fluosilicic acid is limited, it is still necessary to provide storage or disposal for the recovered acid.
One alternative use for fluosilicic acid is disclosed in U.S. Pat. No. 4,557,915 to Nineuil entitled "Production of Phosphoric Acid". In this patent phosphoric acid is mixed with fluosilicic acid after which the acids are reacted with phosphate rock in the production of phosphoric acid. Unfortunately, this process requires that the fluosilicic acid always be mixed with the phosphoric acid and thereby increases the capital cost of the equipment associated with the process in manufacturing phosphoric acid. An additional prior art reference of interest is U.S. Pat. No. 2,636,806 to Ernest Winter entitled "Acidulations of Phosphate Rock".
Other prior art processes for producing phosphoric acid utilizing fluosilicic acid have been proposed, however such processes have not adequately dealt with nor been successful at removing fluorides which are commercially useful such as in the form of hydrogen fluorides at the same time phosphoric acid is generated. In British patent 2,094,282A a process for reacting phosphate rock with fluosilicic acid is disclosed wherein the phosphate and fluorine content of the rock is solubilized in a slurry which is filtered to obtain calcium silicofluoride, as a residue, and a product phosphoric acid. The calcium silicofluoride is further treated with a portion of the phosphoric acid, sulfuric acid, and water to regenerate fluosilicic acid.
In U.S. Pat. No. 1,313,379 to Hachenbleikner a process is disclosed for producing phosphoric acid which includes reacting finely ground phosphate rock with a mixture of dilute hydrofluosilicic acid and hydrofluoric acid containing gelatinous hydrosilicic acid. In the patent, it is stated that the dilute phosphoric acid produced using the process is easily filtered from insoluble materials. However, and as discussed in U.S. Pat. No. 2,636,806 to Winter, it has been determined that such filtering is not possible. The patent to Hechenbleikner also does not provide for recovering fluorides which may be further treated to produce hydrogen fluoride.
U.S. Pat. No. 2,728,634 to Miller does disclose a method of recovering fluorine evolved from the acidulation of phosphate rock. In the process, fluosilicic acid is reacted with ammonia and thereafter the insoluble silica is readily separated from the insoluble ammonium fluoride. Such process, therefore, is dependent upon the use of ammonia in the treatment process and there is no appreciation that insoluble silica can be physically separated from solid fluoride salts, such as calcium fluoride, in order to realize a maximum recovery of hydrogen fluoride during the production of wet process phosphoric acid from fluosilicic acid and phosphate rock.
Additional patents of interest with respect to the production of wet process phosphoric acid from fluosilicic acid and phosphate rock and for recovering hydrogen fluoride are U.S. Pat. Nos. 4,557,915 to Nineui, 3,825,655 to Eipeltaner and 2,636,806 to Winter.